Emissive display technologies, including displays based on cathode-ray tubes (CRTs) and plasma excitation of phosphors have become very popular within many applications. This is typically due to the fact that these technologies natively have superior performance characteristics over reflective or transmissive display technologies, such as displays produced using liquid crystals (LCDs). Among the superior characteristics of these displays is higher dynamic range, wider viewing angle, and, often, lower power consumption. The power consumption of emissive display technologies, however, is directly dependent upon the signal that is input to the display device since the typical emissive display will require almost no power to produce a black image but a significantly higher power to produce a highly luminous white image. More recently, organic light emitting diodes (OLEDs) have been discussed for use in displays and other light emitting devices. Like CRTs and plasma displays, devices constructed based on OLEDs are emissive and have the characteristic that power consumption is dependent upon the input signal.
It is known to control the power of an emissive display by controlling the input signal to the display. For example, U.S. Pat. No. 6,380,943 entitled “Color Display Apparatus”, US 2001/0035850 entitled “Image reproducing method, image display apparatus and picture signal compensation device”. US 2003/0085905 entitled “Control apparatus and method for image display”, US 2001/0000217 entitled “Display Apparatus”. US 2003/0122494 entitled “Driving Device for Plasma Display Panel” all discuss methods for controlling the power of an emissive display, generally plasma displays, wherein the power is estimated for each field or frame of an image signal and the data signal is scaled as a function of some estimate of the average field or frame power to control the overall power of the emissive display. The primary goals of the methods described within these disclosures are to reduce the peak power requirements of the display devices and/or to control the heat that is generated within these display devices. However, these disclosures do not address the fact that active matrix electroluminescent displays such as OLED displays use a driving arrangement that is significantly different in structure than is applied in plasma displays and therefore require a different approach to power reduction to avoid imaging artifacts while reducing the power of the display device.
In a typical voltage-driven active matrix OLED, a pixel driving circuit is provided that regulates the current provided to each OLED within the display device based upon a separate data voltage signal. The current supplied to the OLED by this pixel driving circuit is also somewhat dependent upon the voltage supplied to the circuit by a pair of power lines, comprising a supply power line and a return power line. Ideally, the voltage supplied by the power lines is will be constant for each pixel driving circuit. However, current is typically provided to a large number of OLEDs by a single pair of power lines. Because the power lines have a finite resistance, an unintended voltage differential is produced that is proportional to the current that is conducted through each power line and the resistance of each power line. Since the unintended voltage differential is positively correlated with current and resistance, the loss of voltage along the power lines will be larger when the lines carry high currents or when the lines have a high resistance. This results in variation in the voltage supplied to each pixel driving circuit along the power lines, and subsequent variation in both the current and luminance of each OLED supplied by the power lines. The phenomenon that produces this unintended voltage differential is commonly referred to as “IR drop”. Further, because the resistance of the power lines increases with length, this IR drop will result in the gradual loss of luminance for OLEDs along the power lines as the distance from the power source increases. This loss of luminance has the potential to create undesirable imaging artifacts. Therefore, there is a need to limit unintended voltage drops to avoid these artifacts. IR drop may also occur in electroluminescent display devices which employ other active matrix drive schemes and can result in undesirable imaging artifacts when using these drive schemes as well.
One method to overcome this problem is to reduce the resistance of the power lines as suggested in US 2004/0004444 entitled “Light emitting panel and light emitting apparatus having the same”. Resistance can be reduced by using more conductive materials or by increasing the cross-sectional area of the power lines. In some cases, a highly conductive plane of material can be used in place of one or more individual power lines to reduce the resistance, but this depends on the structure of the device, and it is not always possible to find materials with sufficient properties and/or methods to produce this plane of material. Similarly, the materials that are available to reduce resistance and the cross-sectional area of individual power lines are often fixed by the manufacturing technology that is available, so it is often not cost effective to reduce the resistance of the power lines. Finally, in larger displays, the power lines are typically longer and there are a larger number of OLEDs connected to each set of lines. The power lines therefore tend to have higher resistance and tend to carry higher currents than those on smaller displays. This often limits the size or luminance of displays that can be produced using OLED technology.
It should further be noted that this effect is reduced when the power efficiency of the OLED display device is improved, because less current is needed to produce a given OLED luminance. Therefore, if methods could be developed to reduce the artifacts that occur as a function of IR drop, it may be possible to employ these methods in conjunction with methods to reduce the power of the OLED display device, such as the use of more efficient subpixels as described in US 2004/0113875 entitled “Color OLED display with improved power efficiency” and US 2005/0212728 also entitled “Color OLED display with improved power efficiency” to produce larger and/or higher luminance OLED displays than can be provided using more conventional RGB technology.
It has been suggested that automatic brightness limits can be imposed on OLED displays to limit their power. U.S. Pat. No. 6,690,117 entitled “Display device having driven-by-current type emissive element” discusses a resistor that is placed between the power source and the power lines of an OLED display device. A current dependent voltage drop then takes place across this resistor, reducing the voltage when high currents are present (i.e., when the display has a high relative luminance). This results in a lower data voltage at every OLED in the display and therefore reduces the current that is required at each OLED at the cost of lower luminance. The voltage drop across this resistor can also be sensed and the contrast of the input signal can be modified, dependent upon the voltage drop. While this technique does reduce the peak currents that must be delivered and therefore limits the voltage drop that can occur across the power lines due to IR drop, this technique does not allow a predictable response at each OLED. In fact, it can actually result in additional undesirable artifacts as some TFTs in the panel may be driven at a voltage level below their saturation region, resulting in a further reduction, and more variability, in the current conducted through the OLEDs for a given data voltage. For this reason, the technique taught, while controlling the power of an active matrix OLED display, can contribute to unintended luminance non-uniformities in the display device, reducing the quality of the image that is displayed.
US20050062696 entitled “Display apparatus and method of a display device for automatically adjusting the optimum brightness under limited power consumption” provides a function similar to U.S. Pat. No. 6,690,117 as a resistor is attached to the cathode which also results in reducing the voltage drop across an OLED in the presence of high currents. This approach does not, however, solve the problems associated with the earlier disclosure and does not provide a method for adjusting the contrast in response to changes in display luminance.
In any digitally implemented automatic brightness level scheme a significant component is the method that is used to estimate the quantity that is to be limited. U.S. Pat. No. 6,380,943 entitled “Color Display Apparatus” particularly discusses a method for controlling the power consumed wherein this method includes a method for estimating the power consumed by a RGB display, which might include a “light emission diode apparatus”. Within the power estimation method, the power consumed by each color channel is calculated individually using different gains and the resulting values are summed to compute the total power. Generally, the method for controlling the power is applied to the entire field or frame of data. This disclosure does recognize that it may be desirable to update a portion of a display device at a time to reduce memory requirements and therefore power may be computed for a sub-region within the display at a time. However, the described methods can still result in objectionable artifact levels as this disclosure does not recognize or propose a solution to the problem that IR drop can be different for different power lines and that different luminance levels may result between light emitting elements driven by neighboring power lines when high current loads are present.
There is a need, therefore, for a method that reduces apparent artifacts in an electroluminescent displays such as an OLED display that can result when driving the display such as to require high current levels along power lines with a finite resistance in order to enable the manufacture of larger and/or brighter displays.